Learn Spanish (Spain): Quiero comer más sano. - I want to eat more healthily.

In Spanish, it is very common to use a masculine singular adjective to describe how an action is done. While sanamente (healthily) is grammatically correct, native speakers usually just say sano in everyday conversation.

If a woman says this, should she say Quiero comer más sana?

No, it stays as sano. Because the word is describing the verb comer (how you eat) rather than describing the person speaking, it acts as an adverb and must stay in the default masculine singular form.

Could I use saludable instead of sano?

Yes! Quiero comer más saludable is also perfectly correct and very common. Both sano and saludable mean healthy, and both can be used in this way to describe how you eat.

Why doesn't comer change its ending to match the speaker?

When you have two verbs working together like this, only the first one is conjugated to match the person doing the action (Quiero for I want). The second verb stays in its dictionary form, which is called the infinitive (comer for to eat).